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DBS alumnus Mark Bee talks about research on vocal communication in frogs

Nov. 19, 2012

Dr. Mark A. Bee (left) and Dr. H. Carl Gerhardt (right) outside Tucker Hall.

Dr. Mark A. Bee (left) poses with his former doctoral advisor Dr. H. Carl Gerhardt outside Tucker Hall following his seminar on vocal communication in frogs.

Professor Mark A. Bee (Ph.D., ’01) gave a seminar for the Division of Biological Sciences on November 13th. Bee leads the Animal Communication Lab and is an associate professor in the Department of Ecology, Evolution, and Behavior at the University of Minnesota. In order to better understand the ways in which humans and animals have solved problems associated with noisy communication environments, Bee studies female mate choice and male-male competition in toads and frogs. For his seminar, he presented recent results related to his research on how female frogs select mates from a chorus of croaking males.

In human studies, this common phenomenon is known as the “cocktail party problem.” The term is used to refer to the difficulty humans have understanding speech in noisy social environments. To illustrate this problem, Bee showed an image of a room crowded with people, all of whom appear to be talking in pairs or groups. Pointing to a man who is clearly leaning forward, he said, “We’ve all been this guy right here who is really straining to hear what his colleagues are saying.”

The cocktail party problem is a really hard problem for the auditory system to solve, said Bee, noting that “while healthy auditory systems can do this, impaired auditory systems – for example, people who use hearing aids or cochlear implant devices – really struggle to solve cocktail party like problems.”

This is not a uniquely human problem either. “Frogs are famous for gathering in large, dense, noisy aggregations where males produce very loud mating calls to attract females and females select their mate amid a high-level of noise,” Bee said. The noise generated by males from different species, as well as competitors of the same species, can make individuals calls undetectable, effectively “masking” them.

It turns out that frogs exploit some of the same acoustic cues that humans use to solve this problem of hearing in complex social environments. Like humans, frogs can detect and identify a signal of interest – in the case of female frogs, the call of a potential mate — when it is spatially separated from the noise. This phenomenon is called “spatial release from masking.” Bee shared several experiments showing that female frogs respond faster, have lower signal recognition thresholds, do better at identifying calls of their own species, and can better discriminate among different types of temporal patterns when the call of interest and noise come from different locations. While humans are able to accomplish spatial release from masking because of the size of their heads (e.g., the ears are far enough apart to detect the different arrival times of a sound wave), Bee suggested that small-bodied frogs may accomplish this same effect using a completely different adaptation — their connected ear drums.

Frogs also may deal with the cocktail-party problem by engaging in so-called “dip listening.” Dip listening, explained Bee, is the ability to catch glimpses of sound during brief “dips” in the level of background noise. This phenomenon, he explained, is one humans also use. “We’ve all been in a situation when we are at a party and suddenly and inexplicably the noise drops to a quiet level and there’s that one guy saying something really obnoxious at that time, and everyone hears it,” Bee shared. “This is an extreme example of our auditory system catching speech sound during a ‘dip’ in the background noise.”

Bee shared results from several behavioral studies designed to assess whether gray tree frogs can pick out the call from a frog of the same species during a dip in the noise produced by a mixed-species chorus. His studies show that female frogs can listen in the dips of slowly fluctuating chorus-like noise. He speculated that dip listening may allow females of some species to catch brief “glimpses” of male calls.

The similarity in how frogs and humans solve the cocktail party problem has some evolutionary implications for the neurophysiological mechanisms of hearing. “Because of the interesting evolutionary history of the vertebrate auditory system, there is some potential that frogs and humans use ancient mechanisms of hearing that have evolved and been carried forward over time,” Bee said. “But frogs and mammals have been evolving independently for a long time, which means that there is a lot of scope for potential novelties to arise in these different lineages.”

By studying the cocktail problem in other animals, scientists may learn some general principles about how natural selection has selected solutions to this problem in different lineages. It is within this scope of potential solutions, said Bee, that we may discover biologically-inspired solutions to human hearing problems.

Bee received his Ph.D. in biological sciences from MU in 2001. His doctoral research focused on vocally mediated neighbor recognition in the North American bullfrog (Rana catesbeiana). His doctoral advisor is world-renowned biologist Dr. H. Carl Gerhardt, Curators’ Professor of Biological Sciences, who warmly introduced Bee at the recent seminar.

In his introductory remarks, Gerhardt noted several of Bee’s honors and accomplishments, including his receipt of both an NSF CAREER Award and NIH RO1 in the same year and appointment as a McKnight Land-Grant Professor. “He has been enormously successful, not just in getting grants but in what grants are for — that is a very sustained, wonderful publication rate of research that is very interesting and important,” Gerhardt said.


Written by: Melody Kroll

Related research strengths:
Behavior, Ecology, Evolution, Neurobiology
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